Check on OSM Help: it is more or less standard not to be able to download an image from the main OSM website. It is very server intensive and these days with nearing 2 million signed-up contributors the main map servers just don't have the downtime from routine tasks to render these requests.

Try to export your SVG or PDF betreten 23:00 and 6:00 o'clock because you recive this error message if the servers are to busy. If the server utilisation is above 25% the server will block your request!